Objective To measure the whole brain and hippocampal volumes by the use of serial MRI images of juvenile rhesus monkeys treated with prenatal injections of dexamethasone and vehicle. ABSTRACT:Using the MRI method established for human hippocampal volume in patients with Cushing's syndrome, we measured the developmental sequela of the whole brain and hippocampal volumes in eight rhesus monkeys who had been given prenatal administration of dexamethasone (dex), (dex, 5, and vehicle, 3). A total of 14 MRI imagings were performed in the above 8 monkeys at ages 1.5 to 5 years. Sexual difference was represented in whole brain volume, but no difference was shown between dex- and vehicle-treated monkeys. The brain volumes in both sexes showed a slight increase from 1.5 years to 5 years; in males, an average of 86. 5 cm3 at 1.5 to 1.6 years and 93.5 at 5 years, and in females, 73.5 at 1.8 to 2 years and 85.1 at 5 years. The average brain volumes from 1.5 to 5 years were 88.2 q 1.94 in males and 72.2 q 0.74 in females. The segmental hippocampal volumes adjusted by whole brain volumes showed no sexual difference; the average volume was 119.6 q 4.5 mm3. However, the hippocampal volumes of dex-treated monkeys were on average 10% smaller than those of vehicle treated monkeys; in the vehicle group, an average of 129.3 q 4.1 and in the dex group, 109.9 q 4.8. In our previous studies, prenatal administration of dex at 132 and 135 gestation days (1.25mg/kg) caused degeneration of hippocampal pyramidal and dentate neurons in the fetal brain examined at 135 and 162 gestation days. MRI imaging studies of postnatal brains revealed that juvenile monkeys treated with prenatal dex showed reduced volume of the hippocampus and the monkeys exhibited hypercortisolemia at both circadian basal and poststress levels. These changes suggest that the hippocampus modulates negative feedback of hypothalamic CRF release. Cognitive and memory studies are planned in these monkeys. Keywords MRI hippocampus volume dexamethasone brain volume